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Materials Science and Engineering B 141 (2007) 76–81

Short communication

A novel low-temperature chemical solution route for straight anddendrite-like ZnO nanostructures

Hui Zhang a, Ning Du a, Jianbo Wu a, Xiangyang Ma a, Deren Yang a,∗,Xiaobin Zhang a,b, Zhiqing Yang c

a State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of Chinab Department of Materials and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

c Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

Received 6 July 2006; received in revised form 8 May 2007; accepted 2 June 2007

bstract

The straight and dendrite-like growths of ZnO have been completely and simply controlled by the status of ZnO seed instead of surfactant,emplate, oriented attachment, and ZnO buffer layer on the substrate in the chemical reaction synthesis of ZnO nanostructures. The monodispersenO seeds, which are prepared by in situ quickly injecting the cool mixed zinc acetate and potassium hydrate ethanol solution into the hotatrix aqueous solution of zinc nitrate hydrate and diethylenetriamine at 95 ◦C, improve the straight growth and lots of uniform, straight, and

ingle-crystalline ZnO nanorods with about 20–30 nm in diameter and 300 nm in length are achieved. While, the aggregated ZnO seeds, whichre prepared by dropwise adding potassium hydrate ethanol solution into zinc acetate ethanol solution at 60 ◦C for 3 h, result in the dendrite-like

rowth and the bur-like ZnO nanostructures consisting of hundreds of nanorods with about 30–50 nm in diameter and several micrometers in lengthre formed. Furthermore, the approach presented here provides a simple, low-cost, environmental-friendly and high efficiency route to synthesizehe high quality ZnO nanorods and bur-like ZnO nanostructures. 2007 Published by Elsevier B.V.

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eywords: ZnO nanostructures; Straight and dendrite-like; Chemical solution r

. Introduction

In recent years, the research interests on nanomaterials haveeen greatly spurred due to their essential significance in sci-nce and potential application in technology [1]. ZnO, asn important semiconductor, has been recognized as one ofhe promising materials in a broad range of high-technologypplications such as optical, acoustical, electronic, and opto-lectronic devices [2]. Since the first report of ultraviolet laserrom the arrayed ZnO nanorods, substantial effort has beenevoted to the development of novel synthetic approach andhe exploitation of novel application for ZnO nanostructures3]. Up to now, many interesting nanostructures of ZnO includ-

ng nanowires/nanorods, nanobelts, nanotubes, nanohelices,anorings, nanobridges, nanonails, and other hierarchical nanos-ructures, have been fabricated by thermal evaporation, chemical

∗ Corresponding author.E-mail address: mseyang@zju.edu.cn (D. Yang).

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apor deposition (CVD), chemical reaction and hydrothermalrocess, which can be classified into vapor phase process andolution route [4]. Moreover, there are many novel or promis-ng applications of ZnO nanostructures to be exploited such aseld emission, nanolaser and waveguide, nanosensor, ultravioletetector and optical switch, superhydrophilia and superhy-rophobic, and so on [5]. Recently, wet chemistry route becomeshe promising approach for the synthesis of ZnO nanostructuresue to not only the advantages of low temperature and highield, but also the significance in synthetic chemistry. In general,traight and dendrite-like growths are two kinds of fundamentalechanisms to fabricate one-dimensional or hierarchical ZnO

anostructures in the solution route, which are controlled byhe internal factor such as polar crystal and oriented attach-

ent or external condition such as surfactant, template, andnO buffer layer on the substrate [6]. However, it is either

nevitable to use toxic or expensive surfactant or difficult forigh yield on the substrate or impossible for high efficiencyn the oriented attachment process. More recently, we report aovel approach, i.e. ZnO seed assisted chemical reaction for

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ectogram-scale synthesis of straight, uniform, ultrathin andingle-crystalline ZnO nanorods at 95 ◦C in the aqueous solu-ion without the assistance of surfactant and template, whichrovides a simple route to control the straight growth of ZnO7].

Compared with our previous report, we develop a more sim-le approach to determine the straight or dendrite-like growth ofnO in the aqueous solution only by the status of ZnO seed at

ow temperature using nontoxic ethanol as solvent in this study.oreover, it can be avoided to spend 3 h on the preparation of

nO seeds using the toxic methanol as solvent. The straightrowth of ZnO is occurred when in situ quickly injecting theool mixed zinc acetate and potassium hydrate ethanol solu-ion into the hot matrix aqueous solution of zinc nitrate hydratend diethylenetriamine at 95 ◦C and lasting for 3 h. While, theendrite-like growth is taken place when quickly injecting thes-synthesis ZnO seeds into the hot matrix aqueous solution of

inc nitrate hydrate and diethylenetriamine at 95 ◦C and last-ng for 3 h, which are prepared by dropwise adding potassiumydrate ethanol solution into zinc acetate ethanol solution at0 ◦C for 3 h.

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ig. 1. Morphological and structural characterizations of ZnO nanostructures controlmage (d). The inset image of (d) corresponds to the Fourier transform power spectru

Engineering B 141 (2007) 76–81 77

. Experimental

Straight growth. (a) Synthesis of ZnO seed solution: 270 mginc acetate dihydrate and 210 mg potassium hydroxide wereissolved in 60 and 65 ml ethanol, respectively, and then, mixedogether at 0 ◦C under stirring; (b) synthesis of ZnO nanorods:.67 g zinc nitrate hydrate and 2.67 g PVA were dissolved in00 ml de-ionized water. Meanwhile 1.26 g diethylenetriamineas dissolved in 200 ml de-ionized water. After 10 min of stir-

ing, two kinds of the above-mentioned solution were mixed.ubsequently, the mixed solution was heated and kept at 95 ◦C

n the 500 ml beaker. Immediately, 30 ml above-mentioned ZnOeed was added into the mixed solution and kept for differentime (2, 10, 30, 60, and 180 min). After the reaction completed,he product was cooled, rinsed with de-ionized water, and dried.

Dendrite-like growth. (a) Synthesis of ZnO seed solution:70 mg zinc acetate dihydrate was dissolved in 60 ml ethanol

t 60 ◦C under stirring. Subsequently, 65 ml potassium hydrox-de ethanol solution with 210 mg potassium hydroxide waslowly added into above-mentioned solution for 3 h; (b) synthe-is of dendrite-like ZnO nanostructures: the synthesis process

led by straight growth: TEM image (a and b), FESEM image (c), and HRTEMm.

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as similar to that of the straight growth except using the as-ynthesized ZnO nanoparticles as seeds.

Characterization. The morphology of the sample was char-cterized by field emission scanning electron microscopeFESEM, FEI SIRION) and transmission electron microscopeTEM, JEM 200 CX 160 kV). High-resolution transmissionlectron microscope (HRTEM) observation was performed onhilips CM30 FEG TEM (300 kV) equipped with a postcol-mn GIF2000 detector. The CL spectra were obtained from theitachi S4200 with Tobin yvon spectrum one and Hamamatsu3302.

. Results and discussion

Fig. 1 shows the morphological and structural characteriza-ions of the ZnO nanostructures controlled by the straight-likerowth mechanism. As can be seen from Fig. 1a and b, lotsf uniform, straight and high quality ZnO nanorods with about0–30 nm in diameter and 300 nm in length have been achieved,hich is confirmed by the FESEM image, as shown in Fig. 1c.he HRTEM image of an individual ZnO nanorod, as shown

n Fig. 1d, clearly reveals that only the high-bright lattice pointan be observed, indicating that the ZnO nanorods are single-rystalline in nature in accordance with the Fourier transformower spectrum inserted in Fig. 1d. Furthermore, The spacingf 0.261 nm between adjacent lattice planes corresponds to the

istance of (0 0 2) planes, indicating that [0 0 1] is the growthirection of the ZnO nanorods.

However, when using ZnO nanoparticles prepared by drop-ise adding potassium hydrate ethanol solution into zinc acetate

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Fig. 2. Morphological characterization of the ZnO nanostructures controlled by

d Engineering B 141 (2007) 76–81

thanol solution at 60 ◦C for 3 h as seed, the dendrite-like growthan be taken place, as shown in Fig. 2. Lots of the bur-like ZnOanostructures consisting of hundreds of nanorods with about0–50 nm in diameter and several micrometers in length haveeen observed from the FESEM images, as shown in Fig. 2a and. After long time ultrasonic treatment, the bur-like ZnO nanos-ructures are not destroyed, indicating that the nanostructuresre not formed by the aggregation, as shown in Fig. 2c and d.

To substantially understand the formation mechanism oftraight and dendrite-like growth of ZnO, the morphology evolu-ion with the time has been investigated, as shown in Fig. 3. Whenuickly injecting the cool mixed zinc acetate and potassiumydrate ethanol solution into the hot matrix aqueous solutionf zinc nitrate hydrate and diethylenetriamine at 95 ◦C and last-ng for 2 min, the uniform, cubic-like and monodisperse ZnOeeds with the size of about 10–20 nm have been formed due tohe reaction of zinc acetate and potassium hydrate, as shown inig. 3a, which is similar to the synthesis of the other monodis-erse nanocrystals [8]. After the reaction for 10 min, the shortnO nanorods have been evolved from the cubic-like ZnO seedsue to the preferential growth along [0 0 1], as shown in Fig. 3b.s the reaction proceeds, the relative long ZnO nanorods haveeen obtained, as shown in Fig. 3c and d (reaction for 30 and0 min, respectively). However, the large and agglomerated ZnOarticles consisting of small ZnO seeds have been achieved, ashown in Fig. 3e, which are prepared by dropwise adding potas-

ium hydrate ethanol solution into zinc acetate ethanol solutiont 60 ◦C for 3 h. When injecting the as-synthesized ZnO seedsnto the hot matrix aqueous solution of zinc nitrate hydratend diethylenetriamine at 95 ◦C and reacting for 2 min, ZnO

dendrite-like growth: FESEM image (a and b) and TEM image (c and d).

H. Zhang et al. / Materials Science and Engineering B 141 (2007) 76–81 79

Fig. 3. TEM images of the ZnO nanostructures controlled by straight and dendrite-like growth for different time—straight growth: (a) 2 min, (b) 10 min, (c) 30 min,(d) 60 min; dendrite-like growth: (e) as-synthesized ZnO seed, (f) 2 min, (g) 10 min, (h) 60 min.

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anoparticles firstly grow up and the sphere-like nanostructuresre formed due to the Ostwald ripening process, as shown inig. 3f. In the further prolonged chemical reaction, the pro-ounced growth of the ZnO nanorods originating from the ZnOeeds in the circumference of the ZnO spheres occurs to form theur-like ZnO nanostructures, as shown in Fig. 3g and h (reactionor 10 and 60 min, respectively). Therefore, the morphology evo-ution of the ZnO nanostructures with the reaction time clearlyndicates that the status of ZnO seeds determines the straight orendrite-like growth. The monodisperse ZnO seeds improve thetraight growth, while the aggregated ZnO seeds result in theendrite-like growth.

Up to now, the growth of ZnO nanostructures has been exten-ively investigated and a wide variety of ZnO nanostructuresave been prepared. It is indicated that the morphology of ZnOanostructures can be controllably tailored by the growth condi-ion such as surfactant, concentration, temperature and so on. Forxample, Zhang et al. fabricated the dendrite-like ZnO nanos-ructures by the CTAB assisted hydrothermal process [4m]. Itas further indicated that the sphere-like precursors, which was

ormed at the initial stage, played the critical role for the for-ation of the dendrite-like ZnO nanostructures [9]. Moreover,eng et al. reported that the straight-like ZnO nanostructuresere formed when the precursor concentration was about 0.01 M

nd the dendrite-like ZnO nanostructures were achieved whenhe precursor concentration was about 0.03 M [10]. It was foundhat many more nuclei were generated, and aggregated and grewnto dendrite-like ZnO nanostructures at a yet higher precursoroncentration. In this study, the straight and dendrite-like ZnOanostructures have been controllably synthesized by the statusf ZnO seeds. From the results in Fig. 3, the monodisperse andggregated ZnO seeds have been respectively fabricated by initu quickly injecting the cool mixed zinc acetate and potassiumydrate ethanol solution into the hot matrix aqueous solution at5 ◦C and dropwise adding potassium hydrate ethanol solutionnto zinc acetate ethanol solution at 60 ◦C for 3 h. Therefore,imilar to the previous report [9,10], the monodisperse ZnOeeds can improve the straight growth, while the aggregatednO seeds can result in the dendrite-like growth. Certainly, thebove explanation for the straight and dendrite-like growth ofnO is somewhat conjectural and phenomenological. Further

nvestigation is required to derive the exact mechanism for thetraight and dendrite-like growth of ZnO in the future work.

The room temperature cathodoluminescence (CL) spectra ofhe straight and dendrite-like ZnO nanostructures are recordeds the curves a and b in Fig. 4. Two broad emissions, i.e. one atbout 380 nm belongs to the band to band emission, the other at00 nm induced by interstitial oxygen, have been observed fromtraight and dendrite-like ZnO nanostructures, in agreement withhe previously report [11] The intensity of the broad band for thetraight ZnO nanorods is much lower than that for the dendrite-ike ZnO nanostructures, indicating that there are much fewerefects in the straight ZnO nanorods.

In summary, a more simple, low-cost, environmental-friendlynd high efficiency approach has been developed to control thetraight or dendrite-like growth of ZnO in the solution. Thetatus of ZnO seeds instead of surfactant, template, oriented

ig. 4. The room temperature CL spectra of the straight (a) and dendrite-likeb) ZnO nanostructures.

ttachment, and ZnO buffer layer on the substrate plays the crit-cal role on the straight or dendrite-like growth of ZnO, i.e. the

onodisperse ZnO seeds improve the straight growth, while theggregated ZnO seeds result in the dendrite-like growth. Cer-ainly, the approach presented here provides a way to synthesizehe high quality ZnO nanorods and bur-like ZnO nanostructures.

cknowledgments

The authors would like to appreciate the financial supportsrom Natural Science Foundation of China (60225010), Keyroject of Chinese Ministry of Education and Program for Newentury Excellent Talents in Universities. Thanks Prof. Youwenang for the TEM and FESEM measurements.

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